1. Technical Field
The present invention relates to earth orbit satellite navigation systems and methods for augmenting the existing Global Positioning System in order to optimize navigation accuracy.
2. Background Art
The United States of America Department of Defense Global Positioning System (GPS) has revolutionized the manner in which the world navigates. The baseline constellation of 24 GPS satellites in six orbital planes, each orbit having four satellites per plane, provides accurate and reliable signals from earth orbiting satellites so that anyone using a GPS receiver can determine their position to within 100 meters horizontal and 150 meters vertical accuracy anywhere on earth at any time. The U.S. military can determine position to within 16 meters using properly keyed receivers. While the availability of GPS accuracy has changed the manner in which the world navigates today, the accuracy is not sufficient to meet certain aviation navigation needs.
Aviation navigation requires improvements in three areas of GPS performance: accuracy, integrity, and continuity. Aviation is dependent on very accurate navigation, especially during the landing phase of flight under conditions of reduced visibility. An airplane landing in a Category I precision approach (an instrument approach to a vertical decision height of not less than 200 feet with at least 1/2 mile horizontal visibility, or the runway's operative touchdown zone and runway centerline lights visible) cannot tolerate a 150 meter vertical error offered by the existing GPS. The vertical accuracy requirement for a Category I precision approach is 7.6 meters. The vertical dimension is the most difficult to determine using satellite navigation due to the relative positioning of the satellites to the aviation user. In addition to improvements in accuracy, aviation users must also trust in the correctness of the data being broadcast from the satellites. This trust is known as signal integrity and requires that the user have timely knowledge of satellites that are transmitting erroneous data so that the user can ignore the corrupted signals. The third requirement for aviation users is for a high assurance of continuity of service. For aviation users to depend solely on a GPS augmentation system there must be a very high degree of reliability to support aircraft that are in-flight.
To increase overall system accuracy, additional satellites are needed to augment the current constellation of GPS satellites. Articles by Kinal, "Performance of the INMARSAT-3 Navigation Augmentation Payloads", 1997, by Pedro, "European Constellation Contribution to GNSS", 1997, and by Pullen, "Optimal Augmentation of GPS using Inexpensive Geosynchronous Navigation Satellites", 1997, describe the current approach of placing augmentation satellites in geostationary earth orbits (GEO) over specific geographic regions of the earth. While this approach provides a regional improvement in the navigational accuracy, it has limited global potential due to the lack of available GEO orbit slots and the low viewing angle of GEO satellites at higher latitudes.
The compilation "Global Positioning System: Theory and Applications Volume 1", 1995, edited by Parkinson and Spilker, describes several methods for augmenting the existing GPS. The first method of augmentation is to add one satellite to each of the six GPS orbital planes. The second method is to add augmentation satellites at the GPS orbit altitude in the plane of the equator. The third method is to add augmentation satellites in geostationary orbits. The fourth method of augmentation is to add satellites in inclined orbits below the GPS orbital altitude. All of these methods are designed to increase the minimum number of satellites in view to improve the receiver tolerance to single-satellite outages. While adding one satellite in view may provide the user with a general improvement in navigational accuracy, none of the four methods described improve the vertical accuracy to the degree necessary for a Category I precision approach.
Several U.S. Patents have been issued for satellite systems that provide regional or global coverage. U.S. Pat. Nos. 4,809,935 and 4,854,527 issued to Draim define orbits which require the minimum number of satellites to maintain one satellite in view from the Northern Hemisphere, the Southern Hemisphere, or both simultaneously. U.S. Pat. Nos. 5,551,624 and 5,433,726 and 5,439,190 and 5,415,368 and 5,415,367 issued to Horstein et al. define satellite communication systems which minimizes propagation time delays, the number of beam-to-beam handovers, and the number of satellite-to-satellite handovers. U.S. Pat. Nos. 5,582,367 and 5,669,585 issued to Castiel et al. describe elliptical orbits which provide preferential satellite coverage based upon geographical location, time of day, or offset from the sun. U.S. Pat. No. 5,788,187 also issued to Castiel et al. describes the placement between satellites and the earth stations they are communicating with so as to avoid the line-of-sight to geostationary satellites. Finally, U.S. Pat. Nos. 5,326,054 issued to Turner defines elliptical orbits which reach apogee over a given longitude five or six times per day. Each of these patents generally concern satellite communication systems where the purpose is to maintain one satellite visible somewhere in the sky to permit point-to-point communications. As a result, these patents fail to resolve the need for improving the navigation accuracy to permit a Category I precision approach.